The application of surface treatment liquids is pertinent to the field of soldering electronic components and circuit boards. In an automated production system for soldering printed wiring assemblies (PWAs-printed wiring boards stuffed with components), for example, it is desirable to apply an oxide reduction solution to prepare the PWAs for soldering. A method of restoring solderability of electronic components by applying an oxide reduction solution is described in U.S. Pat. No. 5,104,494 issued Apr. 14, 1992. A further description of reducing agents and their regeneration is provided in U.S. Pat. No. 5,304,297 issued Apr. 19, 1994. The teachings of these prior patents are incorporated herein by reference. In a "fluxless" soldering system (i.e., one in which an oxide removing agent is not present during the actual soldering operation), the reduction solution application system must be compatible with the PWA conveyer system, must allow sufficient bottom clearance for the electronic components and leads, and should minimize the amount of treatment solution applied to the top side of the PWA (where it is more difficult to remove). A method and apparatus for applying a reduction treatment solution by forming a "wave" of the fluid is described in U.S. Pat. No. 5,162,082 issued on Nov. 10, 1992, the teachings of which are incorporated herein by reference.
A need has been identified for improved soldering systems. The desirability of a continuous surface treatment and fluxless soldering operation, along with space and compatibility requirements of PWA conveyor systems, have led to difficulties in conveying PWAs to the surface of a treatment liquid contained in a reservoir. In addition, there is a desire to minimize oxygen concentration in the atmosphere around the PWAs during oxide reduction treatment to avoid excessive discharge of the reducing agent (which would lower the effectiveness of the reduction treatment) and to avoid reoxidation of the treated PWAs before soldering.